Method of processing CDMA signal components

ABSTRACT

The invention relates to a method of processing multipath-propagated components of a signal in a communications system. In the method, a signal transmitted on the radio channel of the communications system is received in a RAKE receiver and an impulse response of the radio channel is formed. In the method, one or more taps having the highest signal energy in the impulse response are located and matched to a short matched filter. A weighting value for the impulse response is calculated on the basis of the one or more taps in the matched filter and a deviation between the weighting value and the center point of the matched filter is compared with a threshold value set for the deviation. The matched filter is moved toward the deviation when the deviation exceeds the threshold value set for the deviation.

This application is the National Phase of International ApplicationPCT/FI00/00739 filed Sep. 1, 2000 which designated the U.S. and that theInternational Application was published under PCT Article 21(2) inEnglish.

FIELD OF THE INVENTION

The invention relates to a radio system and a method of processingmultipath-propagated signal components. The invention relates inparticular to a RAKE receiver of a radio system implemented by a spreadspectrum technique.

BACKGROUND OF THE INVENTION

In radio systems, such as mobile systems, a radio signal between amobile phone and a base station propagates along several routes betweena transmitter and a receiver. The signal may propagate directly from themobile phone to the base station, provided there are no obstaclesbetween them. In urban environments, buildings, cars and other obstaclescause reflection and scattering of the radio signal.Multipath-propagated components of a signal may thus propagate distancesof various lengths on the radio path, resulting in that the componentsarrive at the receiver at different times. Some radio systems, such asradio systems implemented by a spread spectrum technique and employingcode division multiple access (CDMA), can utilize thismultipath-propagation. In such a case, the receiver receives eachmultipath-propagated signal component, and by amplifying and combiningthe components, the transmitted original signal can be betteridentified.

In CDMA, each signal comprises an individual spreading code whichmodulates the baseband while spreading the frequency band of the datasignal. Data signals of several users are transmitted simultaneously onone and the same frequency band and the users are distinguished by thespreading code. Correlators in receivers synchronize themselves to adesired signal which they identify by the spreading code, and return thefrequency band of the signal to the original one. Signals, which containanother spreading code, arriving at a receiver do not, in an idealsituation, correlate but keep their wide frequency band and are thusreceived as noise in the receivers. The aim is to select the spreadingcodes used by the system so that they are orthogonal with respect toeach other, i.e. do not correlate with each other. One user can have oneor more spreading codes depending on the required transmission capacity.

A RAKE receiver made up of one or more RAKE fingers, i.e. correlators,is generally used as a CDMA receiver. RAKE fingers are independentreceiver units whose task is to despread and demodulate one receivedmultipath-propagated signal component. In addition to the RAKE fingersintended for receiving signals, a CDMA receiver typically has at leastone separate searcher whose task is to search for the various signalcomponents transmitted with a desired spreading code, to identify theirphases and to allocate the signal components to the RAKE fingers. Thesearcher is implemented according to prior art, for instance by means ofa matched filter (MF). In practice, the length of a matched filter in asearcher finger is 256 spreading code units, i.e. chips, because thephase of the received signal is not known. Each RAKE finger can bedirected to correlate with a signal component propagated along adifferent route, each component arriving at the receiver delayed in aslightly different manner. The RAKE fingers are directed by giving thecorrelator information on the desired spreading code and its phase.

After starting to receive a signal, according to prior art, a RAKEfinger keeps its spreading code synchronized to the incoming signal ofthe finger using the known early-late code tracking loop, for instance.The receiver then has three correlators: one tracks the incoming signalexactly synchronized to it, a second synchronizes itself to the earlyphase of the spreading code, which is a phase preceding the currentphase by half a chip, for instance, and a third synchronizes itself tothe late phase which is a phase delayed by half a chip from the currentphase.

A receiver solution in which each RAKE finger tracks the changes in thedelay of its own code phase, has significant drawbacks, because inconnection with RAKE fingers, the implementation of the correlators andtracking the delay increase considerably the complexity of theimplementation of the finger. A further considerable drawback in priorart solutions is that when the fingers track independently their signalcomponents propagating in different directions, the components have atendency to merge, whereby two different fingers synchronize themselvesto the same spreading code phase. One prior art solution, a matchedfilter of a traffic channel, which is relatively long, has increased thecomplexity of the receiver of the searcher on account of the requiredcomputational power, thus also increasing the requirements set on theequipment.

BRIEF DESCRIPTION OF THE INVENTION

It is an object of the invention to implement an improved method andapparatus for processing a multipath-propagated signal in a CDMA radiosystem. This is achieved by a method, described in the following, ofprocessing multipath-propagated signal components in a communicationssystem, in which a signal transmitted on a radio channel of thecommunications system is received in a RAKE receiver and an impulseresponse is formed in a delay estimator of the RAKE receiver on thebasis of the received signal by correlating the received signal with afirst matched filter. In the method, one or more impulse response tapshaving the highest signal energy is located, said one or more taps arematched to a second matched filter which is shorter than the firstmatched filter, a weighting value is calculated on the basis of the oneor more taps in the second matched filter for the impulse response usingstatistical methods, a deviation between the weighting value and acentre point of the second matched filter is compared with a thresholdvalue set for the deviation, the second matched filter is moved forwardwhen the deviation exceeds the threshold value set for exceeding thedeviation, the second matched filter is moved backward when thedeviation undershoots the threshold value set for undershooting thedeviation, said four last steps are repeated during the reception of thesignal.

The invention also relates to a RAKE receiver in a communicationssystem, which receiver comprises means for receiving a signaltransmitted on a radio channel of the communications system, one or moredelay estimators for forming an impulse response of the radio channel onthe basis of the received signal by correlating the received signal witha first matched filter, and one or more correlators for tracking amultipath-propagated component of the received signal. The RAKE receiverfurther comprises means for locating one or more taps of the impulseresponse having the highest signal energy, means for matching said oneor more taps to a second matched filter which is shorter than the firstmatched filter, means for comparing a deviation between a weightingvalue and a centre point of the second matched filter with a thresholdvalue set for the deviation, means for moving the second matched filterforward when the deviation exceeds the threshold value set for exceedingthe deviation, means for moving the second matched filter backward whenthe deviation undershoots the threshold value set for undershooting thedeviation, and means for repeating said four last steps during thereception of the signal.

It is an object of the invention to remove the problems related to theneed for a long matched filter in the reception of a traffic channel ofa radio receiver implemented by a spread spectrum technique. A furtherobject of the invention is to simplify the operation of the fingers,i.e. correlators, of the receiver to such an extent that they need nottrack the phase of their own spreading code.

In radio systems implemented by a spread spectrum technique andemploying code division multiple access, receivers can utilizemultipath-propagated signal components in such a manner that thecomponents are received with different delays and combined, whereby theoriginal signal can be amplified. The present invention relates to theabove-mentioned receivers, without, however, being restricted to themultiple access method being a pure CDMA, the multiple access method canalso be a TDMA or FDMA combined with the CDMA.

In a preferred embodiment, the invention can be implemented in aRAKE-type receiver having one or more searchers, i.e. delay estimators,and one or more fingers. The task of a searcher is to find themultipath-propagated signal components and their delays and to allocatethe signal components in question to correlators which track thespreading code phases allocated to them. One task of a searcher infinding the multipath-propagated components is to find the correct codephase by means of a matched filter. A signal is received as input intothe matched filter and samples are taken from it. The samples arecorrelated with predefined data, such as a part of a spreading code. Theinput signal multiplied by a spreading code is obtained as output fromthe matched filter. The spreading code used in the matched filter mustbe relatively long, 256 spreading code units, or chips, for instance, ona random access channel (RACH) in which a terminal within the servicearea of a cellular radio network transmits a connection request to thenetwork. As a result of the contact on the random access channel, thereceiver is able to synchronize itself to the signal, and consequently,it can use a shorter section of the spreading code, which speeds up theprocessing of the information received on the traffic channel. Further,an impulse response for the radio channel is formed in the searcher toestimate the interference caused by the radio path to the signal. Theimpulse response can be formed by means of pilot symbols transmitted onthe channel and known by the receiver. How and on the basis of whichinformation the impulse response of the channel is formed in thesearcher, is not essential for the invention. The impulse responsedepicts the signal energy of the multipath-propagated components and thedelay of the components. On the basis of the found delays, the bestsignal components depicted by the impulse response are allocatedaccording to prior art to be tracked by the correlators, of which thereare preferably 1 to 5, but it is also possible to have more of them inthe receiver.

The basic idea of the invention is to calculate a weighting value forthe impulse response on the basis of the impulse response of thechannel. According to one embodiment, the weighting value is a weightedaverage of the impulse response of a short matched filter in such amanner that the value to be weighted is the location of the impulseresponse tap, or index, and the weight is the strength of the tap, i.e.signal strength. The centre of gravity of the impulse response can bedefined on the basis of the weighted average so that it makes the use ofa short matched filter possible. In an embodiment, the spreading code ofa matched filter used in the reception of a traffic channel is 32 units,or chips.

One embodiment of the invention further involves controlling the delaysof the code phases of the fingers according to the centre of gravity ofthe impulse response. In such a case, the fingers do not independentlytrack the changes occurring in the delays of their signal components,but the searcher informs all fingers of a change in the centre ofgravity of the impulse response, whereby the fingers can change theirown timing accordingly.

The invention provides several advantages. Using a short matched filterin the searcher in the reception of the traffic channel reducesconsiderably the calculations required in timing the receiver to thespreading code. The implementation of the fingers is also significantlysimplified when the fingers need not track the delays of their signalcomponents themselves.

BRIEF DESCRIPTION OF THE FIGURES

In the following, the invention will be described by means of preferredembodiments and with reference to the attached drawings in which

FIG. 1 shows the principle of a UMTS mobile telephone system,

FIG. 2 shows a UMTS mobile telephone system depicted by means of a GSMnetwork,

FIG. 3 shows the operation of a transmitter-receiver pair,

FIG. 4 shows the structure of a RAKE receiver,

FIG. 5 shows a channel estimate matched to a short matched filter, and

FIG. 6 shows a flow chart of an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention can be used preferably in different mobile telephonesystems implemented by a spread spectrum technique, and the invention ishere described in a universal mobile telephony system (UMTS) employingwideband code division multiple access, without limiting the inventionto it, however. The examples presented in the description of theinvention are based on the description of the Wideband CDMA system.

The structure of the UMTS is described by means of FIGS. 1 and 2. FIG. 1shows the structure of the UMTS on a rough level, so it is clarified bymeans of FIG. 2 by showing which part of the known GSM systemcorresponds approximately to which part of the UMTS. It is clear thatthe presented comparison is in no way binding, but provides a guideline,since the responsibilities and functions of the different parts of theUMTS are still being designed. The figures only contain the blocks thatare essential for describing the invention, but it is obvious to aperson skilled in the art that a conventional mobile telephone systemalso contains other functions and structures which need not be describedin more detail herein.

The UMTS is as shown in FIG. 1 divided structurally into system parts,the main division being between the terminal and infrastructure. In thiscontext, a terminal refers to a mobile telephone, a portable computer ora domestic appliance adapted to a telecommunications network. A terminalcan further be divided into two subsections, mobile equipment ME and auser services identity module USIM, the interface between them beingcalled Cu. The mobile equipment ME performs the facilities of the radiointerface and also contains a number of other facilities, such asconnecting the mobile equipment to a portable computer. The user serviceidentity module contains data and functions for identifying users in theradio system. The USIM also makes it possible for the user to change theused terminal in the manner known from the SIM card of the GSM system.The infrastructure subsection is divided into an access network (AN)domain and a core network domain, the interface between them beingcalled Iu. The access network (AN) domain, also called the UTRAN (UMTSterrestrial radio network), comprises physical equipment and mechanismswith which the user can use the network, whereas the core network domainis responsible for network management at a higher level, for instancemanagement of user location information, data transmission andsignalling. The core network domain is divided into three subsections, aserving network SN, a home network HN and a transit network TN. Theserving network SN handles call routing and user data transmissionbetween the information source and destination. The serving network SNis also connected to the home network HN and transit network TN. Thehome network HN handles network functions which are based on a permanentlocation. The transit network TN handles connections outside the UMTSnetwork in the cases where the other party of the connection residesoutside the UMTS network.

According to FIG. 2, a circuit-switched connection can be establishedfrom the user equipment UE to a subscriber terminal 200 connected to apublic switched telephone network PSTN 202. A base station 220 has amultiplexer 214, transceivers 216 and a control unit 218 which controlsthe operation of the transceivers 216 and the multiplexer 214. With themultiplexer 214, the traffic and control channels used by severaltransceivers 216 are placed on a transmission link lub which is aninterface between a base station and a serving switching centre. Thetransceivers 216 of the base station 220 are connected to an antennaunit with which a bi-directional radio link Uu is implemented to thesubscriber equipment Ue. The antenna unit of a base station is usuallyimplemented with at least one diversity antenna, in which case anuplink, for instance, has two locally separate antenna branchesutilizing the diversity of a signal.

A telephone exchange, i.e. a group switch, 210 is connected to a controlunit 212 which typically manages the following: radio resources, controlof handovers between cells, power control, timing and synchronization,and paging subscriber terminals. The group switch 210 is used to switchspeech and data and to connect signalling circuits. The base stationsystem 220 also comprises a transcoder 208 usually located as close aspossible to a mobile switching centre 206, because speech can then betransmitted between the mobile switching centre 206 and the group switch210 in mobile system format, thus saving transmission capacity. Thetranscoder 208 transforms different digital speech coding formats usedbetween a public telephone network and a radio telephone network to suiteach other, for instance from the 64 kbit/s format of a fixed network toa cellular radio network format (e.g. 13 kbit/s) and vice versa. In FIG.2, the core network domain is made up of an infrastructure external tothe UTRAN and belonging to a mobile telephone system. Of the apparatusesof the core network domain, FIG. 2 shows the mobile switching centre 206and a gateway mobile switching centre 204 which handles the connectionsof the mobile telephone system to an external telecommunicationsnetwork, in this case the public telephone network 202.

The radio interface Uu between the user equipment UE and the UTRAN is athree-layered protocol stack whose layers are a physical layer L1, adata link layer L2 and a network layer L3. The L2 is further dividedinto two sub-layers, a link access control layer LAC and a medium accesscontrol layer MAC. The network layer L3 and the LAC are further dividedinto control (C) and user (U) layers. The physical layer L1 providesdata transmission services for transport channels, the MAC and higherlayers. The L2/MAC layer transmits information between physicaltransport channels and logical channels higher in the protocol stack.There are different types of logical channels, such as control channelsand traffic channels, in the UMTS system like in other digital radiosystems. Some of the radio channels are in the uplink direction from theuser equipment to the cellular radio system, whereas some are in thedownlink direction from the mobile telephone system to the userequipment. On a control channel, no radio resources are reserved for theuser equipment, but control channels handle matters related to the useof the system, such as paging user equipment on a common paging channelPCH. In the uplink direction, one control channel acts as a randomaccess channel (RACH) on which the user equipment requests connectionestablishment from the network. Radio resources are reserved for actualtraffic channels for the user equipment depending on the datatransmission need. One logical traffic channel is a dedicated channelDCH on which information is transmitted from the radio system to theuser equipment. A UMTS radio system comprises numerous other channels,too, but their description herein is not essential for the invention.

Frame and burst structures used on the physical channels differ fromeach other depending on which physical channel they are transmitted. Aframe refers here to an entity including several bursts, in which casethe second time-slot in each frame can, for instance, be reserved forone user for transmitting the burst. One example of a frame is the PDPCHphysical channel frame of the UMTS time division duplex (TTD) mode,whose length is 10 milliseconds and the frame is divided into sixteentime-slots each having a length of 0.625 milliseconds. A data packettransmitted in a time-slot is called a burst. One burst described abovecan contain 2560 chips of information with chips 0 to 1103 containingdata, chips 1104 to 1359 containing a midamble, chips 1360 to 2463containing again data and at the end of the burst, there is a 96 chipslong guard period. The data in the midamble is often also called atraining sequence or a pilot.

A training sequence is a number of symbols known by the receiver and itis transmitted to the user equipment from the network on a forwardaccess channel FACH, for instance, prior to the actual connectionestablishment. This training sequence received by the user equipment canbe used during the connection in both downlink and uplink direction, butdifferent training sequences can also be used for the differenttransmission directions. When receiving bursts on a channel, a receiver,which can be user equipment or a radio network base station, makes achannel estimate, i.e. impulse response, on the basis of the trainingsequence. Making a channel estimate means that the receiver tries toestimate how the radio path has distorted the data contents of theburst. On the basis of the received information, the receiver can, usingknown methods, try to correct the data contents of the burst accordingto the channel estimate. By means of the training sequence and theimpulse response formed from it, the channel quality can be estimatedusing known methods, such as the C/I ratio (carrier/interference), SIR(signal interference ratio), bit error rate, or by examining the ratioof the chip energy to the interference power density E_(c)/I₀.

The following describes by means of FIG. 3 the operation of a radiotransmitter—radio receiver pair on a general level. The radiotransmitter can reside in a base station or in user equipment UE, andthe radio receiver can also reside in user equipment UE or in a basestation. The top part of FIG. 3 shows the essential functions of theradio transmitter in such a manner that the processing steps of thecontrol channel are on top and below them are the processing steps ofthe data channel before the channels are combined and transmitted to thephysical channel of the radio link. Pilot bits which form the trainingsequence of the burst and which the receiver uses in channel estimation,are located in the control channel 314. User data 300 is located in thedata channel. The channels are channel-coded in blocks 302A and 302B.Different block codes, for instance, are channel coding, one examplebeing the cyclic redundancy check (CRC). In addition, convolution codingis typically used and its different variations, such as puncturedconvolution coding or turbo coding. Pilot bits are, however, notchannel-coded, because the intention is to find out the distortionscaused by the channel to the signal. When the different channels havebeen channel-coded, they are interleaved in an interleaver 304A, 304B.The purpose of the interleaving is to facilitate error correction. Ininterleaving, the bits of difference services are scrambled in a certainway together so that a momentary fade on the radio path does notnecessarily yet make the transmitted information unidentifiable. Theinterleaved bits are spread with a spreading code in blocks 306A, 306B.The chips obtained in this manner are scrambled with a scrambling codeand modulated in block 308, and the separate signals received fromdifferent channels are also combined in block 308 for transmissionthrough the same transmitter. Finally, the combined signal is taken toradio frequency parts 310 which may comprise different power amplifiersand filters restricting bandwidth. A closed loop control used intransmission power control usually controls a transmission power controlamplifier in this block. An analogue radio signal is transmitted throughan antenna 312 to the radio path Uu.

The bottom part of FIG. 3 shows the essential functions of the radioreceiver. The radio receiver is typically a RAKE receiver whoseoperation is described in connection with the description of theinvention in FIG. 4. An analogue radio frequency signal is received fromthe radio path Uu with an antenna 332. The signal is taken to radiofrequency parts 330 which comprise a filter preventing frequenciesoutside the desired frequency band. After this, the signal is convertedto an intermediate frequency in block 328 or directly to baseband, andthe converted signal is sampled and quantized. Because this is amultipath-propagated signal, the signal component propagated alongdifferent routes are combined, if possible, in block 328 which accordingto prior art comprises the actual RAKE fingers of the receiver. Theinterleaving of the received physical channel is removed indeinterleaving means 326 and the deinterleaved physical channel isdivided in a demultiplexer 324 into data streams of different channels.The channels are directed each to its own channel-decoding block 322A,322B in which the channel coding used in transmission, for instanceblock coding and convolution coding, is decoded. Each transmittedchannel can then be taken to any necessary further processing. Thesystem control channels are taken to a control part 336 of the radioreceiver.

The information to be transmitted to a radio channel is multiplied by aspreading code, thus spreading a relatively narrowband information to awide frequency band. Each link Uu has its own spreading code or codeswith which the receiver identifies transmissions intended for it.Typically, there are a maximum of 256 different mutually orthogonalspreading codes in use simultaneously. For instance, if the UMTS uses afive megahertz carrier at the speed of 4.096 megachips per second in thedownlink direction, the spreading factor 256 corresponds to atransmission speed of 32 kbit/s, and respectively, the highest practicaltransmission speed is achieved by spreading factor 4, whereby the datatransmission speed is 2048 kbit/s. Accordingly, the transmission speedon the channel varies stepwise from 32, 64, 128, 256, 512, 1024 to 2048kbit/s, the spreading factor being 256, 128, 64, 32, 16, 8 and 4,respectively. The data transmission speed at the user's disposal dependson the channel coding used. For instance, if ⅓ convolution coding isused, the user's data transmission speed is about one third of the datatransmission speed of the channel. The spreading factor indicates thelength of the spreading code. For instance, the spreading codecorresponding to the spreading factor one is (1). The spreading factortwo has two mutually orthogonal spreading codes (1,1) and (1,−1).Further, the spreading factor four has four mutually orthogonalspreading codes: below an upper level spreading code (1,1), there arespreading codes (1,1,1,1) and (1,1,−1,−1), and below a second upperlevel spreading code (1,−1), there are spreading codes (1,−1,1,−1) and(1,−1,−1,1). The formation of spreading codes is continued in this waywhen moving to lower levels of a code tree. The spreading codes of agiven level are always mutually orthogonal. Likewise, a spreading codeof a given level is orthogonal to all the spreading codes derived fromanother spreading code of the same level to the next levels. Intransmission, one symbol is multiplied by a spreading code, whereby thedata spreads to the used frequency band. For instance, when using thespreading code 256, 256 chips represent one symbol. Correspondingly,when using the spreading code 16, 16 chips represent one symbol.

FIG. 4 shows the parts of one embodiment of a RAKE receiver that areessential for the invention. A conventional CDMA receiver generally has1 to 5 RAKE fingers, each listening for one multipath-propagatedcomponent of a received signal. Due to the mobile nature of a radiotelephone, the propagation environment between a base station and theradio telephone varies continuously and the strength and number of themultipath-propagated signals change with the location of the radiotelephone. With reference to FIG. 4, the received signal is taken in theRAKE receiver after an antenna receiver 400 and radio frequency parts402 to an A/D converter 404. The multipath-propagated components of thereceived signal are found in the searcher 406 by forming an impulseresponse for the channel. The searcher defines the delays of thedifferent signal components from the impulse response and allocates thecomponents to the RAKE receiver fingers 408A to 408D to track. Formingthe impulse response is done according to prior art and is not essentialfor the invention. In one embodiment, the searcher 406 tracks thequality of the connection according to a known quality assessmentmethod, such as the bit error rate (BER) and re-allocates the fingers408A to 408D, if the BER decreases below a pre-set threshold value. Onealternative is to re-allocate the fingers regularly. During connectionestablishment, a correlator in the searcher 406 tries to synchronizeitself to the training sequence spread by a spreading code to form theimpulse response. For a burst received on a RACH channel, for instance,a relatively long matched filter, such as a 256 chips long filter, isused in the searcher so that the correct phase of the spreading codewould be found as quickly as possible. After having synchronized to thecorrect phase, the matched filter of the searcher can be shortened to 32chips, for instance, for traffic channel reception. The signals receivedfrom different correlators are combined in a combiner 410, after whichthe signal is transmitted to a decoding part 412. FIG. 4 shows theessential parts of the structure of an antenna receiver having oneantenna branch. It is obvious that antenna diversity can be utilized inan antenna receiver of a base station, in which case there are more thanone antenna branches.

The parts of operation of the RAKE receiver shown in FIG. 4 that areessential for the invention are described next by means of FIGS. 5 and6. In the starting step 600 of FIG. 6, a base station receiver of anembodiment of the invention is ready to receive a RACH burst on a RACHchannel from a mobile phone located within the service area of the basestation. In step 602, the signal arrives at the receiver whose searchercorrelates with the signal by means of a long matched filter. Theimpulse response of the channel is also formed in the receiver on thebasis of the received signal according to step 604. Taps having thehighest energy are found from the impulse response pattern according tostep 606, according to which taps, a short matched filter is adapted fortraffic channel reception in step 608. FIG. 5 shows an example of animpulse response pattern which is matched to a 32 chips long shortmatched filter 500. The Y axis 502 shows the signal energy where value 1represents the maximum energy. The X axis 504 shows the length of thematched filter in chips, and in the example, it is 32 chips. The curve506 thus represents the energy of the impulse response in proportion tothe location of the matched filter in chips. The figure shows that theimpulse response pattern has two high peaks, one of which is marked as508. It is essential for the invention to keep the two peaks in FIG. 5,for instance, inside the short matched filter 500, in which case theshort matched filter can be used in traffic channel reception. The useof a short matched filter is advantageous, because the shorter thefilter, the less correlation calculations are needed in the receiverduring signal reception. According to the invention, a weighting valueof the impulse response is calculated on the basis of the impulseresponse pattern in the short matched filter, using methods ofstatistical analysis. With reference to step 610 of FIG. 6, said methodof statistical analysis refers, according to a preferred embodiment ofthe invention, to the calculation of a weighted average of the impulseresponse according to formula (1), $\begin{matrix}{{C_{g} = \frac{\sum\limits_{k = 1}^{N_{ir}}{k \cdot R_{k}}}{\sum\limits_{k = 1}^{N_{ir}}R_{k}}},{wherein}} & (1)\end{matrix}$C_(g) is the centre of gravity of the impulse response, R_(k) is theenergy of the impulse response tap, k is the index of the impulseresponse tap and N_(ir) is the length of the impulse response window.According to an embodiment of the invention, the centre of gravity ofthe impulse response can be calculated at intervals of 10 ms, forinstance, after which the centre of gravity can, as in FIG. 5, be movedto index point 16 when the length of the matched filter is 32 chips.Formula (1) shows that all the different index values of the shortmatched filter are used in calculating the centre of gravity, wherebythe effect of low taps, i.e. noise, is relatively high. According to oneembodiment of the invention, only the taps which have the highest energyare included in calculating the centre of gravity, in FIG. 5, forinstance, only two taps would be included in the calculation. Accordingto one embodiment, the centre of gravity can also be calculated as anaverage of the delays of the allocated RAKE fingers. Then the strengthof the signal at a certain moment does not affect the calculation, butonly the fact whether a decision has been made to allocate a RAKE fingerfor the signal component. According to step 612 of FIG. 6, in oneembodiment of the invention, a threshold value N_(s) is set for aweighting value change, which it should exceed before the location ofthe second matched filter is changed. After calculating the weightingvalue, the change in the weighting value is compared with the thresholdvalue N_(s) and if${C_{g} < {\frac{N_{ir}}{2} - \frac{N_{ir}}{N_{s}}}},$the code phase is changed N_(s) chips backwards according to step 614,and if ${C_{g} > {\frac{N_{ir}}{2} + \frac{N_{ir}}{N_{s}}}},$the code phase is changed N_(s) chips forward according to step 616. Inthe example in FIG. 5, it is possible to estimate that the centre ofgravity of the impulse response is approximately 26, in which case${{26 > {\frac{32}{2} + \frac{32}{8}}} = 20},$when the threshold value N_(s) is 8. In this case, the matched filterwould be moved 8 steps forward. According to step 618, the calculationof the weighting value is done at certain predefined intervals and thecalculation is continued as long as the signal is received.

According to a preferred embodiment of the invention, the fingercorrelators are controlled according to the centre of gravity changes inthe RAKE receiver. When the centre of gravity of an impulse responsepattern in a short matched filter changes one chip backward, forinstance, the searcher informs all fingers that they should also changethe code phase they track one chip backward.

The invention is preferably implemented by program, in which case thebase station 220 is a microprocessor, and the means implementing themethod of the invention are implemented as a software in it. Theinvention can naturally also be implemented by hardware providing thenecessary functionality, for instance as an ASIC (application-specificintegrated circuit) or using separate logic components.

Even though the invention has been explained in the above with referenceto an example in accordance with the accompanying drawings, it isobvious that the invention is not restricted to it but can be modifiedin many ways within the scope of the inventive idea disclosed in theattached claims.

1. A method of processing multipath-propagated signal components in acommunications system, in which a signal transmitted on a radio channelof the communications system is received in a RAKE receiver and animpulse response of the radio channel is formed in a delay estimator ofthe RAKE receiver on the basis of the received signal by correlating thereceived signal with a first matched filter, the method comprising:locating one or more taps having the highest signal energy in theimpulse response, matching the one or more taps to a second matchedfilter shorter than the first matched filter, calculating a weightingvalue for the impulse response using statistical methods on the basis ofthe one or more taps in the second matched filter, comparing a deviationbetween the weighting value and a centre point of the second matchedfilter with a threshold value set for the deviation, moving the secondmatched filter forward when the deviation exceeds the threshold valueset for exceeding the deviation, moving the second matched filterbackward when the deviation undershoots the threshold value set forundershooting the deviation, and repeating the calculating a weightingvalue, comparing a deviation, moving the second matched filter forwardand moving the second matched filter backward during reception of thesignal.
 2. The method of claim 1, further comprising: defining a delayof each multipath-propagated signal component on the basis of theimpulse response, allocating a correlator of the RAKE receiver toreceive at least one multipath-propagated signal component and advisingto the correlator the delay of the multipath-propagated signalcomponent, receiving the signal in the correlator while taking intoconsideration the delay of the multipath-propagated signal component, ifthe weighting value changes, changing the delay advised to thecorrelator according to the change in the weighting value, and repeatingthe changing the delay if the weighting value changes always when theweighting value is calculated.
 3. The method of claim 1 or 2, whereinthe weighting value is calculated as an average of allocated correlatordelays.
 4. The method of claim 1 or 2, wherein the weighting value iscalculated as a weighted average of the impulse response taps in such amanner that the value to be weighted is the location of the impulseresponse tap and the weight is the energy amount in the tap, accordingto the formula:$C_{g} = \frac{\sum\limits_{k = 1}^{N_{ir}}\;{k \cdot R_{k}}}{\sum\limits_{k = 1}^{N_{ir}}\; R_{k}}$wherein C_(g) is the weighting value, R_(k) is the energy of the impulseresponse taps, k is the index of impulse response taps, and N_(ir) isthe length of an impulse response window.
 5. The method of claim 1 or 2,further comprising setting for the weighting value deviation a referencevalue N_(s) indicating the smallest possible movement of the code phaseof the second matched filter from the centre point of the second matchedfilter, and wherein: moving the second matched filter backward compriseschanging the location of the second matched filter N_(s) code phasesbackward, if$C_{g}\langle {\frac{N_{ir}}{2} - \frac{N_{ir}}{N_{s}}} $wherein C_(g) is the weighting value and N_(ir) is the length of animpulse response window, moving the second matched filter forwardcomprises changing the location of the second matched filter N_(s) codephases forward, if${{ C_{g} \rangle\frac{N_{ir}}{2}} + \frac{N_{ir}}{N_{s}}},{and}$repeating the last two steps always when the weighting value iscalculated.
 6. The method of claim 1 or 2, wherein the communicationssystem is a cellular radio network implemented by a spread spectrumtechnique and employing code division multiple access (CDMA).
 7. A RAKEreceiver in a communications system, which receiver comprises: means forreceiving a signal transmitted on a radio channel of the communicationssystem, one or more delay estimators for forming an impulse response ofthe radio channel on the basis of the received signal by correlating thereceived signal with a first matched filter, one or more correlators fortracking a multipath-propagated component of the received signal, meansfor locating one or more taps having the highest signal energy in theimpulse response, means for matching the one or more taps to a secondmatched filter which is shorter than the first matched filter, means forcomparing a deviation between a weighting value and a centre point ofthe second matched filter with a threshold value set for the deviation,means for moving the second matched filter onward when the deviationexceeds the threshold value set for exceeding the deviation, means formoving the second matched filter backward when the deviation undershootsthe threshold value set for undershooting the deviation, and means forrepeating the last four steps during the reception of the signal.
 8. TheRAKE receiver of claim 7, further comprising: means for defining a delayof each multipath-propagated signal component on the basis of theimpulse response, means for allocating a RAKE receiver correlator forreceiving at least one multipath-propagated component and advising tothe correlator the delay of the multipath-propagated component, meansfor receiving the signal in the correlator while taking intoconsideration the delay of the signal component, if the weighting valuechanges, means for changing the delay advised to the one or morecorrelators according to the change in the weighting value, and meansfor repeating the last step always when the weighting value iscalculated.
 9. The RAKE receiver of claim 7 or 8, further comprisingmeans for calculating the weighting value as an average of the delays ofthe allocated correlators.
 10. The RAKE receiver of claim 7 or 8,further comprising: means for calculating the weighting value as aweighted average of the impulse response taps in such a manner that thevalue to be weighted is the location of the impulse response tap and theweight is the energy amount in the tap, according to the formula:$C_{g} = \frac{\sum\limits_{k = 1}^{N_{ir}}\;{k \cdot R_{k}}}{\sum\limits_{k = 1}^{N_{ir}}\; R_{k}}$wherein C_(g) is the weighting value, R_(k) is the energy of the impulseresponse taps, k is the index of impulse response taps, and N_(1r) isthe length of an impulse response window.
 11. The RAKE receiver of claim7 or 8 further comprising means for setting for the weighting valuechange a reference value N_(s) indicating the smallest possible movementof the code phase of the matched filter, and wherein: means for movingthe second matched filter backward comprises means for changing thelocation of the second matched filter by N_(s) code phases backward, if$C_{g}\langle {{\frac{N_{ir}}{2} - \frac{N_{ir}}{N_{s}}},} $wherein C_(g) is the weighting value and N_(ir) is the length of animpulse response window, means for moving the second matched filterforward comprises means for changing the location of the second matchedfilter by N_(s) code phases forward, if${{ C_{g} \rangle\frac{N_{ir}}{2}} + \frac{N_{ir}}{N_{s}}},{and}$means for repeating the last two steps always when the weighting valueis calculated.
 12. The RAKE receiver of claim 7 or 8, wherein thecommunications system is a cellular radio network implemented by aspread spectrum technique and employing code division multiple access(CDMA).
 13. A computer program product including computer program codeto cause a microprocessor to perform a method of processingmultipath-propagated signal components in a communications system, inwhich a signal transmitted on a radio channel of the communicationssystem is received in a RAKE receiver and an impulse response of theradio channel is formed in a delay estimator of the RAKE receiver on thebasis of the received signal by correlating the received signal with afirst matched filter, the method comprising: locating one or more tapshaving the highest signal energy in the impulse response, matching theone or more taps to a second matched filter shorter than the firstmatched filter, calculating a weighting value for the impulse responseusing statistical methods on the basis of the one or more taps in thesecond matched filter, comparing a deviation between the weighting valueand a centre point of the second matched filter with a threshold valueset for the deviation, moving the second matched filter forward when thedeviation exceeds the threshold value set for exceeding the deviation,moving the second matched filter backward when the deviation undershootsthe threshold value set for undershooting the deviation, and repeatingthe calculating a weighting value, comparing a deviation, moving thesecond matched filter forward and moving the second matched filterbackward during reception of the signal.
 14. The computer programproduct of claim 13, wherein the method further comprises: defining adelay of each multipath-propagated signal component on the basis of theimpulse response, allocating a correlator of the RAKE receiver toreceive at least one multipath-propagated signal component and advisingto the correlator the delay of the multipath-propagated signalcomponent, receiving the signal in the correlator while taking intoconsideration the delay of the multipath-propagated signal component, ifthe weighting value changes, changing the delay advised to thecorrelator according to the change in the weighting value, and repeatingthe changing the delay if the weighting value changes always when theweighting value is calculated.
 15. The computer program product of claim13 or 14, wherein the weighting value is calculated as an average ofallocated correlator delays.
 16. The computer program product of claim13 or 14, wherein the weighting value is calculated as a weightedaverage of the impulse response taps in such a manner that the value tobe weighted is the location of the impulse response tap and the weightis the energy amount in the tap, according to the formula:$C_{g} = \frac{\sum\limits_{k = 1}^{N_{ir}}{k \cdot R_{k}}}{\sum\limits_{k = 1}^{N_{ir}}R_{k}}$wherein C_(g) is the weighting value, R_(k) is the energy of the impulseresponse taps, k is the index of impulse response taps, and N_(ir) isthe length of an impulse response window.
 17. The computer programproduct of claim 13 or 14, the method further comprising setting for theweighting value deviation a reference value N_(s) indicating thesmallest possible movement of the code phase of the second matchedfilter from the centre point of the second matched filter, and wherein:moving the second matched filter backward comprises changing thelocation of the second matched filter N_(s) code phases backward, if$C_{g} < {\frac{N_{ir}}{2} - \frac{N_{ir}}{N_{s}}}$ wherein C_(g) is theweighting value and N_(ir) is the length of an impulse response window,moving the second matched filter forward comprises changing the locationof the second matched filter N_(s) code phases forward, if${C_{g} > {\frac{N_{ir}}{2} + \frac{N_{ir}}{N_{s}}}},{and}$ repeatingthe last two steps always when the weighting value is calculated. 18.The computer program product of claim 13 or 14, wherein thecommunications system is a cellular radio network implemented by aspread spectrum technique and employing code division multiple access(CDMA).